1. Field of Invention
This invention relates generally to the field of wireless communication networks and more specifically to a method of dynamically allocating bandwidth within such a network while maintaining a minimum guaranteed bandwidth.
2. Description of the Related Prior Art
FIG. 1 shows a wireless communication system 8 which includes a base transceiver station (BTS) 10 connected to a network 12 which may be, for example, an Asynchronous Transfer Mode (ATM) network. The BTS 10 can be an ATM switch such as the 7470 MSP™ manufactured by Alcatel Networks Corporation, configured with one or more radio interface cards, or the like. Through radio frequency (RF) transmission, BTS 10 communicates with one or more network interface units 14. NIUs 14 in turn, are hard wired using copper wire, fibre optic cable or the like to a plurality of client hardware platforms such as a Private Branch Exchange (PBX) 16, Local Area Network (LAN) 18, a desktop terminal, or similar devices. The system is designed to support all forms of traffic including video, audio and bulk data transmissions.
As indicated above, the base transceiver stations 10 are interlinked to network interface units 14 via RF links. One frequency is used for transmission from the BTS 10 to the NIU 14 (the “downlink” frequency), and a different frequency is used for transmission from the NIU 14 to the BTS 10 (the “uplink” frequency). In a typical configuration, microwave transmissions facilities are used operating in the 20 to 28 gigahertz range. The downlink is usually of a broadcast nature, while the uplink uses a Time Division Multiple Access (“TDMA”) technique to allow NIUs 14 to share the frequency allotted. The TDMA technique divides up the total bandwidth into a predetermined number of time slots, which are allocated to NIUs using various schemes which will be discussed below.
In a TDMA wireless system, NIUs are connected to base transceiver stations by way of wireless RF links. A diagram depicting the system architecture of this arrangement is shown at FIG. 2 where an ATM system is used as an example and AAL1=ATM Adaptation Layer Type 1, AAL5=ATM Adaptation Layer Type 5, and CE=Circuit Emulation. In this example, traffic arriving from the Ethernet port and CE port is segmented into ATM cells using AAL5 and AAL1 respectively. The arriving traffic streams are then mapped onto ATM virtual channels (VC). The ATM cells are queued per service category and or per VC, and served by the scheduler. The data transmitted from NIUs to the BTS over the wireless link is formatted in accordance with TDM formatting. The uplink consists of time slots which are grouped into frames. Each NIU transmitting data via the connection monitors the system timing and transmits data in the time slots allocated to it. Each frame contains 133 time slots and each frame is 5.625 ms in duration. Each time slot can be independently assigned to an NIU and can be used to send a single ATM cell. Each time slot falls into one of the following categories: polling slot (P); contention slot (C); guard slot (G); and reservation slot. Reservation slots are further divided into dynamic bandwidth allocation (DBA) and guaranteed bandwidth allocation (GBA). In the downstream direction, information is broadcast to the NIUs. The downlink consists of MPEG packets, two such packets comprising a total of seven ATM cells. The first packet in each frame is considered the frame start packet and is used by the Medium Access Control (MAC) layer to provide information to the NIUs. The speed of the downlink is approximately 41 Mbps (ATM layer). The speed of the TDMA uplink is approximately 10 Mbps (ATM layer). Both the uplink and the downlink frames have the same duration, and they are synchronized with their frame starts offset typically by ½ frame (See FIG. 3).
A radio link can support one or more virtual connections, such as ATM virtual channel and virtual path communications or IP traffic flows. Referring to the uplink transmission, the time slots may be either permanently assigned to a connection for the duration of the connection or dynamically assigned to a connection on demand. As discussed above, the former is referred to as guaranteed bandwidth allocation (GBA) and the latter as dynamic bandwidth allocation (DBA).
In the GBA mode, when the connections are created, a number of dedicated time slots are assigned to individual NIU's based on the traffic parameters of the connections. The time slots which are dedicated to a NIU are always accessible to the NIU and cannot be shared by other NIUs even when those time slots are idle. Therefore, the subscribed bandwidth is guaranteed. A algorithm or rule is usually used to select the time slots. For example, the algorithm disclosed in U.S. patent application Ser. No. 09/244,165 entitled “Method And Apparatus For Controlling Traffic Flows In A Packet-Switched Network”, filed on Feb. 4, 1999 may be used to select the time slots which minimize the jitter.
In the DBA mode, time slots are not dedicated to any NIU. Instead, a pool of time slots are shared among multiple NIUs. If a NIU has data to send, i.e., there are cells/packets in its queue, the NIU sends data through the assigned time slots. In the current methodology, there is a single data queue in the NIU. The queue is checked at the beginning of every 5.625 ms frame. If the queue is not empty and no request or grant is in progress, the NIU sends a bandwidth request to the BTS through a contention slot. The request contains the queue occupancy information, e.g., in terms of cells or bytes. A random access algorithm, namely Ternary Tree Splitting with Free Access which is well known in the art, is used in determining the contention slot in which the request is to be sent. At the BTS, a time slot allocation algorithm, for example the one proposed in U.S. patent application Ser. No. 09/316,439 entitled “Method And Apparatus For Assigning Time Slots Within A TDMA Transmission”, filed on May 21, 1999, is used to allocate time slots. The NIU sends the cells/packets through the assigned time slots. Partial grant is allowed and subsequent requests can be made to request for more bandwidth.
The advantage of the GBA is that the subscribed bandwidth is always guaranteed. However, GBA is often inefficient in terms of bandwidth utilization when the traffic is bursty, i.e., during the idle period the bandwidth is wasted.
The advantage of DBA is that the bandwidth is allocated on demand. DBA is efficient in terms of bandwidth utilization when the traffic is bursty. However, since the time slots are shared among multiple NIUs and the current DBA algorithm assigned time slots on a “best effort” basis, the required bandwidth may not be guaranteed when multiple NIUs contend for bandwidth.
Given the limitations of GBA and DBA described above, a need exists for a method of allocating bandwidth to an NIU on demand while ensuring that a minimum amount of bandwidth is always available to each NIU.